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URADING AND SCREENING                              227
should fall ncmri'st to the pulley and as the distance from the pulley increases sumwMvdy fiiHT particles should come to rest. If the particles are of different kiiulnof material mid with different modes of fracturing, this mode of grading may offer means for separating the constituents by screening or other ways following the nation obtained by the bolt travel. If the particles be of varying K{XH.*ifie gravity their alight ing position for those of the same size will be modified ki HOHW extent by this difference, the heavier particles alighting somewhat closer to the pulley: but npeeifie gravity differences will not affect the grading so much as differeiH'eN in si/e and shape.
V, IL Hirnhfel<P has experimented with this mode of grading in endeavoring to «liHfov«»r a means of separation for various constituents of a low value ore which did not differ much from out4 another in point of specific gravity. He found that very high bt'lt-Hfteed was neeeHnary and this required that the apparatus be massive and haw go«Ki foundations. Another difficulty encountered was with the differential crawl of flit" material on the bell under the high speed necessary. This was overcome by having two belts in i-ontact with one another, both running in the same direction and with the same speed, the material being conveyed by the lower one and slippage tn*IitjK prevented by the upper. Both heltH wen*, run horizontally.
Rolling to Eliminate Defective Shots or Other Small Roundish Objects.— This is done mi an inelinrd flat. Hurfa.ee the* perfect and defective shot being intro-dueed at the upper end. At the bottom there are two receptacles for receiving the* t wo kind.* of i-hot, The outer one, since the perfect shot roll faster and have a flutter trajectory after leaving the edge of the inclined plane, receives the perfect sboi and the one nearest to the rolling surface the imperfect ones. If instead iif the Ntatinnary enlace a movable one were used such as an inclined endless 1 ii'lf gradation* in perfect inn of sphere could be obtained.
Grading by Volume. The first patent of record for a distinctive device of thl Kind i-- the one issued to McKesson and Rice, for theirScrecnlessSizer (U. S. !*af. UMl,or»7, Nov. Ill, I'.M'J) but earlier patents embody the principle of this Mm* In ;i certain extent. The early Kit finger ore separation table which made its ;nivejit abniit 1 s."».'t eon--i-.ted <>f a. .slightly inclined rectangular deck to which motion wa-. given parallel t»» one of the sides. The motion, was of a differential rlmniHer and (ended to advance any material fed on the dock in a direction parallel to the motion of the table but owing to the inc.liiuition the particles tended to fur; ue a diagonal coiir.-e. The material was fcul at a corner of the deck
The un»drni \\ilflry table invented in lS(.)f> and others of siinihir type which have appeared «m ih«- inarKet .• incc and nil a tremendous improvement on the parent liittmjji-r patent nnpl..v a plane differentially reciprocating deck on which arc mounted :t piiiraht \ «.l' jiarallc-1 uillr «»r channels, parnllel to Mu^ line of motion given the deck. "IheM- nil!*- tap»-r t'roiu thrir gr«':ite.-.it dcptli at the feed (Mid of the deck where the iMech:mi"W proihicing advancing motion in pla.ccd, to the lower end where they arc of I«iif lit!!•• tfri-atrr than paper depth or thickness. All the machines referred to arc for e.inci-iiUafmK or*' in water. < hi material of uniform spcc.ifie, gnivity such as a mass of Imi-lv Kr..'in.i ipiarf/. the \Villl«-\ and other tables of a similar type give a gradation from coar-i- :if f h«- motion merhanr m end of the table to line at the lower end.
Tin- ni!!« would have %ery lit fie effect on the action of the machine used in this way other lh:ui that thev :-erve to keep the grains suspended while the grading operation is takiuK place. Without tin- riffles the water would tend to leave the mass^of grains f«» ri more ,,r !e>s ili^ree leaving sticky immobile hanks of material. The